IPhone 2g to iPhone 4?

When you upgrade nothing gets changed in your plan unless you tell them to or it's required. The existing data plans will stay grandfathered in so you would just need a data plan for the iPhone that will replace the Blackberry which would be the $15 200mb plan or the $25 2gb plan. So nothing will change except for whatever data plan you choose for the one phone that will require you add it.